Working culture (not conditions) are a factor indeed. We are driven to have more and therefore work more. This also means having less room for children.
But family institutions are less stable now then they have been before. I blame overwork for this, and also commercialism that has taken away needs of a nurtured family. This is not a govt problem.
I blame people's expectations, and also women's rights.
People point to poor nations, or the Great Depression, as times or places when people had/have more kids. But peoples' expectations are different there. People during the Depression didn't expect to send their kids to college, for instance; usually, they just put them to work on the farm, or in factories. These days, we don't allow child labor, and want kids to get an education for some strange reason, and that all costs money.
Women's rights and contraception are also a big issue. A century ago, women didn't have any rights. Their husbands could beat them if they got drunk and mad, and it was OK. Women couldn't have real jobs. Basically they were slaves. They could (usually) choose whom to marry, but it was a crapshoot, and divorce was usually unobtainable, plus women couldn't survive on their own very well. Plus contraception wasn't very good. These days, for some strange reason, this has changed and women now have rights, and aren't stuck being slaves to their husbands, so they have the ability to choose whether to marry at all (!), and how many kids to have, if any.
If we want people to have lots of kids again, we need to make women 2nd-class citizens without the ability to have gainful employment, we have to ban good contraception and divorce, we have to allow spousal abuse, and we have to allow children to be forced to work in dangerous and dirty factories.
The reason is called social progress. Even though some people are reactionary and idolise "the golden age", whenever they imagine it being, in general most developed societies have seen massive social progress. Rolling it back so that women can get back to being baby-making machines is... impractical and cruel.
In many western countries the reason why people aren't having as many kids as before is because it's too expensive, and because housing is too expensive. You aren't going to have children, if you're still living in your parent's basement, or sharing an apt with roommates.
Childcare in the US, for example, is outrageously expensive. If you aren't lucky enough to have a grandparent or other relative do it for free, or for nominal compensation, your childcare cost is prohibitively expensive. In many cases it's so expensive, that it's more cost effective for the parent with lower salary to quit and stay home. Hopefully programs like the free childcare New York City is offering become more common.
Housing costs prohibit lots of things including getting married (unless the wife is also to work). Lack of social institutions destruct having children.
When these two problems are combined, a more socialist mindset would burden govt to resolve with subsidized housing and childcare. This is not a sustainable social system.
But family institutions are less stable now then they have been before. I blame overwork for this, and also commercialism that has taken away needs of a nurtured family. This is not a govt problem.